Television service providers typically multiplex a large number of channels onto a single transport medium. The content for these channels is usually compressed into one or more Moving Picture Experts Group (MPEG) standard transport streams, with each channel being represented by a different program identifier (PID) contained in each data packet. Each transport stream is, in turn, modulated onto a respective carrier wave within a dedicated region of bandwidth around that carrier wave. Each region of bandwidth is sometimes referred to as a “QAM,” which also stands for “quadrature amplitude modulation,” because that is a typical carrier modulation technique used. In fact, the term QAM is variously used to refer to the carrier modulation technique, the region of bandwidth, and even equipment used to perform the modulation or otherwise process the modulated transport stream.
Each QAM bandwidth region typically has a fixed maximum bandwidth capacity and contains a subset group of the channels. For instance, a particular QAM bandwidth region may contain two or more high-definition channels per QAM bandwidth region, or ten to twenty standard definition channels. Regardless of how the channels, or PIDs, are divided amongst the QAM bandwidth regions, the total bandwidth of the channels carried on a QAM bandwidth region is not supposed to exceed the total fixed capacity of that QAM bandwidth region. However, the total bandwidth of a group of channels is difficult to determine, because the actual bandwidth needed by each channel can vary widely and unpredictably over time. Factors that affect the variability in bandwidth needed by a channel include, for instance, the amount of motion in a video scene or the number of quick cuts between scenes.
Therefore, while television service providers would like to maximize the number of channels on each QAM bandwidth region, they are limited somewhat by unpredictable variations in actual bandwidth used by the channels. If the actual bandwidth exceeds the QAM bandwidth region capacity, then the quality of the content may temporarily drop for all MPEG streams contained within the same QAM bandwidth region, resulting in artifacts such as blocking and freezing of video and distortion of audio. The provider can put fewer channels on each QAM bandwidth region to reduce the probability of exceeding QAM bandwidth region capacity. However, this is not ideal because a portion of bandwidth in that region will go unused most of the time. To deal with this problem, many providers implement an MPEG groomer, which further compresses the content for each channel on a QAM bandwidth region at times when the total bandwidth needed by the channels on that QAM bandwidth region would otherwise exceed the capacity of that bandwidth region. This additional compression (sometimes called “recompression”) is typically lossy, meaning that information is irreversibly removed from the transmitted data so that the data may be smaller. Therefore, such recompression causes an inevitable drop in content quality as received by the customer/television viewer. While the recompression quality drop is expected to be smaller and more controllable than the quality drop that would occur by simply letting the actual bandwidth exceed the capacity of the QAM bandwidth region, such a quality drop is still undesirable.